Wind turbines are vulnerable to being struck by lightning; sometimes on the tower, nacelle and the rotor hub, but most commonly on the blades of the turbine. A lightning strike event has the potential to cause physical damage to the turbine blades and also electrical damage to the internal control systems of the wind turbine. Wind turbines are often installed in wide open spaces which makes lightning strikes a common occurrence. Accordingly, in recent years much effort has been made by wind turbine manufacturers to design wind turbines so that they are able to manage effectively the energy imparted to them during a lightning strike in order to avoid damage to the blade and the cost associated with turbine down-time during blade replacement.
In general, lightning protection systems for wind turbine blades are known. In one example, an electrically conductive lightning receptor element is arranged on an outer surface of the blade to receive a lighting strike. Since the receptor element is electrically conductive, lightning is more likely to attach to the receptor element in preference to the relatively non-conductive material of the blade. The receptor element is connected to a cable or ‘down conductor’ that extends inside the blade to the root and from there connects via an armature arrangement to a charge transfer route in the hub, nacelle and tower to a ground potential. Such a lightning protection system therefore allows lightning to be channelled from the blade to a ground potential safely, thereby minimising the risk of damage. However, the discrete receptors are relatively complex to install during fabrication of the blade and, moreover, they leave a significant portion of blade area exposed to a risk of lightning strike.
Such a receptor arrangement provides discrete conductive points to which lightning may attach. To increase the effectiveness of such a system, US2011/0182731 describes a wind turbine blade having a conductive layer that is laid over the outer surface of the blade shell so as to make contact with the receptor elements. The conductive layer increases the area of the blade that can receive lightning, thereby increasing the rate at which the receptor elements can capture lightning strikes safely. Although a conductive layer used in this way can be said to increase the capability of the lightning protection system to intercept lightning strikes, such a system can be complex to manufacture since the conductive layer must be added to the blade after the blade shell has been fabricated. This requires an additional time-consuming manufacturing step thereby increasing assembly time and cost.
It is against this context that the invention has been devised.